Receiver pipe

ABSTRACT

A receiver tube ( 1 ) for a solar trough collector which focuses the solar radiation incident thereon towards a focal line includes an absorber tube ( 2 ) which in the mounted condition of the receiver tube ( 1 ) extends on the focal line of the trough collector and which is in the form of part of a flow path for a heat carrier medium, and a gas-tightly closed casing tube, in the internal space ( 8 ) of which the absorber tube extends, wherein the flow path for the heat carrier medium is passed outwardly through the wall of the casing tube in the end regions thereof. It is provided that the casing tube includes at least two case portions ( 4, 5 ) which in the mounted condition extend in the direction of the focal line and are gas-tightly connected together along generatrices, the first case portion ( 4 ) comprises a material which is transmissive for the solar radiation, the second case portion ( 5 ) comprises a fracture-resistant material, and the flow path for the heat carrier medium is passed outwardly through a part of the wall of the casing tube, that is not formed by the first case portion.

The invention concerns a receiver tube for a solar trough collector of the kind set forth in the classifying portion of claim 1.

Solar power plants are known in which sunlight is converted into heat by means of a multiplicity of parabolic trough collectors. Those trough collectors involve mirrors which are shaped parabolically with a single axis and which are arranged in long rows and each of which focuses the solar radiation incident thereon onto a focal line in which an absorber tube forming the central component of a receiver tube is arranged and is connected to the absorber tubes of the adjacent mirrors in order in that way to form a flow path for a heat carrier medium (generally a heat carrier oil) which, after having been heated in the absorber tube arrangement to up to 400° C. goes to a heat exchanger in which steam is generated, driving a generator for electric current by way of a turbine. In other variants steam is heat directly in the absorber tube arrangement.

In addition to the absorber tube the receiver tube includes a gas-tightly closed casing tube which surrounds the absorber tube and is evacuated or is filled with a gas which is a poor heat conductor in order to thermally insulate the absorber tube. The absorber tube is passed out of the casing tube at the end regions thereof to be able to make a flow communication with the absorber tubes of the adjacent mirrors.

A receiver tube of the kind just described can be found for example from DE 103 05 428 A1.

To be able to achieve a power of for example 50 megawatts with a trough collector solar power plant, parabolic mirrors are used with a mirror surface area of about 360,000 square meters, which are equipped with 15,000 receiver tubes involving a total length of 60 kilometers.

A disadvantage with the known receiver tubes is that their absolutely gas-tightly closed casing tubes comprise glass and therefore have a certain tendency to fracture, which can have the result that, within a short time, a large number of receiver tubes lose thermal insulation and have to be replaced, because otherwise the efficiency of the plant suffers excessively. As, during such an exchange operation which involves an interruption in the flow path for the heat carrier medium, at least parts of the plant have to be shut down, such an exchange operation is carried out only at times of low or no solar radiation, in particular in the evening or at night. That involves on the one hand additional complication and expenditure and on the other hand means that the plant can only continue to be operated at a reduced level of efficiency, until there is a period of time which is appropriate for the exchange operation.

In comparison the object of the invention is to develop a receiver tube of the kind set forth in the opening part of this specification, such that it can be inexpensively produced and easily fitted and by virtue of a substantially improved resistance to fracture, has an increased service life, whereby the frequency of exchange operations is considerably reduced.

To attain that object the invention provides the features recited in claim 1.

Because the casing tube has two case portions which are connected together preferably by a heat-resistant silicone adhesive and of which only the first comprises a comparatively fragile material while the second is made from a break-resistant material, in particular metal and particularly preferably high-quality steel, the strength of the receiver tube is considerably increased because the break-resistant case portion can carry practically all forces which act on the receiver tube in production, upon storage, transport, upon assembly and during operation. The first, less break-resistant case portion practically represents a window which is carried and stabilized by the second case portion and which is just so great that in the mounted condition the solar radiation reflected by the associated trough collector can pass through it into the interior of the casing tube and can be incident on the absorber tube. The through-passage means through the casing tube, which are required for connecting the absorber tube to the absorber tubes of the adjacent receiver tubes, can be made in the second case portion which, particularly when it comprises metal, can readily carry the forces which occur for example upon changes in length caused by temperature fluctuations, so that in contrast it is possible to dispense with the length compensation bellows which are required in the state of the art and which are connected to the ends of the absorber tube and which are generally welded into the ends of the casing tube that comprises glass, at least when correspondingly great wall thicknesses are employed. If that is unwanted, such compensating bellows can also be used in a receiver tube according to the invention.

A particular advantage of a receiver tube according to the invention is that, even when breakage of the first, radiation-transmissive case portion generally consisting of an inexpensive glass occurs, there is no need to completely replace the receiver tube in question. Rather, without interrupting operation of the plant, the broken first case portion can be released from the second case portion which remains at its position of installation and which carries the absorber tube and can be replaced by a new “glass window”.

A further problem with solar power plants with trough collectors is that the latter are frequently not exposed over their entire length to the same ambient conditions, because of their considerable extent. Thus, different air flows mean that locally limited cooling of the collector material can occur, which does not involve the entire collector, so that that can entail local deformation phenomena and thus deviations from the ideal parabolic shape. Such deviations can also occur due to non-optimum mounting of the collectors or have to be minimized in plants corresponding to the state of the art by a very high level of structural and work-related technical complication and expenditure.

In any event it is to be assumed that the focal lines of most trough collectors do not ideally lie completely on the associated absorber tube at least at times or even permanently. To overcome such focusing errors DE 103 05 428 A1 proposes producing on the casing tube which consists of glass, a structure which focuses the sunlight on the absorber tube arranged in the casing tube by way of diffraction and/or refraction. That structuring of the casing tube wall however results in locally differing material thicknesses and makes the production of such a glass tube considerably more expensive and difficult. In addition DE 103 05 428 A1 mentions other secondary concentrators which serve to overcome the focusing errors and which for example involve a polished metal sheet which is fixed in the interior of the casing tube either thereon or on the absorber tube. That kind of secondary concentrators is identified as being disadvantageous because they lead to shadowing of the absorber tube and thus a worsening of the level of efficiency. In addition such a sheet represents an additional component, the production and fitting of which increase the costs of a receiver tube.

To resolve that problem, in accordance with a particularly advantageous development of the receiver tube according to the invention, it is provided that the second case portion is so shaped and designed that it acts as a secondary concentrator, that is to say it focuses the solar radiation incident thereon towards the absorber tube because the solar radiation is not thrown onto the absorber tube by the associated trough collector.

For that purpose the second case portion, on its concave inside, can either have a smooth surface which is adapted to be reflecting in particular by mirroring, or structures can be provided on that inside, which lead to a deviation from the smooth surface and which are so designed that they exert a light deflection function in such a way that they deflect sunlight onto the absorber tube, the energy content of which would otherwise be lost, with their reflecting and in particular mirrored surfaces. As the second case portion comprises a material which is easy to work, in particular metal, the above-mentioned structures which can be raised or recessed can be easily and inexpensively produced in one piece thereon or can be fixed thereto. Additional components as are mentioned in the above-discussed state of the art are not required in that case. The slight shadowing of the absorber tube in relation to solar radiation incident from the exterior on the second case portion can be tolerated as the slight loss in energy yield which is caused thereby is by far outweighed by the other advantages.

In conventional receiver tubes, whose casing tube comprises a one-piece, gas-tightly closed glass cylinder, there is in addition the problem that with the passage of time diffusion processes in which for example hydrogen originating from the heat carrier oil passes through the absorber tube into the interior of the casing tube mean that the existing vacuum is worsened or the thermal conductivity of the gas present there is increased. In addition substances diffusing out of the absorption layer of the absorber tube can deploy such an effect. It is admittedly known from the state of the art to arrange catchers for the hydrogen and/or other troublesome substances in the interior of the casing tube in order to prolong the service life of the receiver tubes. Because of the additional components that also results in an increase in production costs. In addition saturation of those catchers occurs some time, whereby the thermal insulation capability of the casing tube is so severely worsened that replacement thereof becomes inevitable, which, as already mentioned hereinbefore, results in operational shutdown and the disadvantages linked thereto, in the case of conventional receiver tubes.

To eliminate those difficulties, a particularly preferred variant of the receiver tube according to the invention provides a valve arrangement extending through the wall of the second case portion from the exterior into the internal space.

That valve arrangement can include either only one valve which can be connected to a gas suction removal pump by way of a suction conduit to restore the vacuum present in the interior of the casing tube. As it is desirable for the valves of a plurality of casing tubes to be connected to a single suction conduit each of those valves is in the form of a non-return valve in order to ensure that upon fracture of one of the casing tubes, the entire suction conduit and all casing tubes connected thereto by way of the above-mentioned valves are not filled with ambient air.

In contrast, for receiver tubes whose casing tube is filled with a heat-insulating gas, for example helium or nitrogen, it is possible to provide two valves of which one serves as a feed valve and the other as a suction valve and is connected to a corresponding conduit. That arrangement makes it possible for the interior of the casing tube to be flushed and charged with fresh gas whenever an excessively large proportion of unwanted substances has accumulated therein. A particular advantage is that the evacuation and flushing operations in question make replacement of the receiver tube in question superfluous and can be carried out in ongoing operation.

Here too the valves of a plurality of casing tubes are always connected to one and the same gas feed conduit and one and the same gas suction conduit. As preferably the operating pressure of the heat-insulating gas both in the casing tubes and also in the gas feed conduit is far below atmospheric pressure (for example of the order of magnitude of 0.05 bars) preferably both valves of each casing tube are in the form of non-return valves for the same reasons as above.

It is advantageously provided that arranged in the interior of the receiver tube is at least one and preferably two temperature sensors with which the temperature of the absorber tube is measured at the feed flow and/or discharge flow end. That makes it possible on the one hand to detect the amount of heat introduced into the heat carrier medium in the receiver tube in question and on the other hand to recognize in good time an excessive rise in temperature to prevent overheating of the absorber tube.

It is further preferred to arrange in the interior of the receiver tube at least one hydrogen sensor, by means of which it is possible to detect if the hydrogen content reaches an excessively high value. That is important because an excessively high hydrogen content worsens the thermal insulation of the absorber tube. If a predeterminable limit value is exceeded here, then in the case of a receiver tube in which there should actually be a vacuum, it is possible to implement gas suction removal by means of the valve provided according to the invention. In the case of the receiver tubes filled with a gas which is a poor conductor of heat, gas flushing can be carried out by means of the two above-mentioned valves to eliminate the hydrogen which has diffused thereinto.

A pressure sensor is also provided in the interior of receiver tubes operated with vacuum, by means of which pressure sensor it is possible to detect a worsening in the vacuum at an early stage in order then to perform a suction operation.

These and further advantageous configurations of a receiver tube according to the invention are set forth in the appendant claims.

The invention is described hereinafter by means of embodiments by way of example with reference to the drawing in which:

FIG. 1 shows a diagrammatic exploded perspective view of the essential components of a first embodiment of a receiver tube according to the invention for a solar trough collector,

FIG. 2 shows the receiver tube of FIG. 1 in the partly assembled condition, wherein the first case portion which is transmissive for the solar radiation is omitted for the sake of clarity,

FIG. 3 shows the receiver tube of FIGS. 1 and 2 in the completely assembled condition,

FIG. 4 shows a view corresponding to FIG. 1 of a second embodiment,

FIG. 5 shows the receiver tube of FIG. 4 in the partly assembled condition, wherein the first case portion which is transmissive for the solar radiation is omitted for the sake of clarity,

FIG. 6 shows the receiver tube of FIGS. 4 and 5 in the completely assembled condition,

FIG. 7 shows a perspective view of a second case portion of a receiver tube according to the invention with fins which extend in the longitudinal direction and which project radially inwardly, of the same radial extent,

FIG. 8 shows a section perpendicularly to the longitudinal axis of the second case portion of FIG. 7,

FIG. 9 shows a perspective view of a second case portion of a receiver tube according to the invention with fins which extend in the longitudinal direction and which project radially inwardly, of the differing radial extents,

FIG. 10 shows a section perpendicularly to the longitudinal axis of the second case portion of FIG. 9,

FIG. 11 shows a perspective view of a short portion of a second case portion of a receiver tube according to the invention with structures of rectangular outline which are repeated in the longitudinal and transverse directions and which project radially inwardly,

FIG. 12 shows a section extending perpendicularly to the longitudinal axis of the second case portion of FIG. 11,

FIG. 13 shows a perspective view of a short portion of a second case portion of a receiver tube according to the invention with structures of polygonal outline which are repeated in the longitudinal and transverse directions and which project radially inwardly,

FIG. 14 shows a section extending perpendicularly to the longitudinal axis of the second case portion of FIG. 13,

FIG. 15 shows a perspective view of a short portion of a second case portion of a receiver tube according to the invention with structures of round outline which are repeated in the longitudinal and transverse directions and which project radially inwardly,

FIG. 16 shows a plan view of the part of the case portion of FIG. 15,

FIG. 17 shows a section extending perpendicularly to the longitudinal axis of the second case portion of FIG. 15, and

FIG. 18 shows a side view of the part of the case portion of FIG. 15.

In the Figures in which the trough collectors and the structures serving to support the receiver tubes have been omitted for the sake of simplicity, the same or mutually corresponding parts are denoted by the same references.

As can be seen from the Figures a receiver tube 1 according to the invention has an elongate absorber tube 2 which in conventional manner is provided with an absorber coating (not shown separately) which serves to convert as large a proportion as possible of the solar radiation focused onto the absorber tube 2 by a trough collector and possibly a secondary concentrator into heat by which a heat carrier medium (typically a heat carrier oil or however also steam) which in operation flows through the absorber tube 2 is heated.

The absorber tube 2 is surrounded over its entire length by a casing tube which according to the invention comprises two case portions 4, 5 which comprise different materials, extend over the entire length of the receiver tube 1 and are gas-tightly connected together along two generatrices for example by means of a heat-resistant silicone adhesive or in any other fashion. Together with two end walls 6, 7 extending perpendicularly to the axis of the receiver tube 1 the two case portions 4, 5 enclose in gas-tight relationship an internal space 8, the volume of which is greater than the volume of the absorber tube 2. To thermally insulate same, the internal space 8 of the casing tube is either evacuated or filled with a gas which forms a poor heat conductor and in particular transports little heat by convection. The longitudinal axes of the casing tube and the absorber tube 2 extend parallel to each other and may but do not necessarily have to coincide; the latter is shown in the Figures.

The first case portion 4 which in the mounted condition of the receiver tube 1 is towards the trough collector comprises a material having a high degree of transparency for solar radiation, in particular glass. In the illustrated embodiments the first case portion 4 is of a cross-section which is curved outwardly in the shape of a circular arc, that is to say convex. It may however be of any other shape, for example in the form of a flat plate or curved inwardly. In addition it may have optical structures which improve focusing of the solar radiation reflected by the trough collector.

The second case portion 5 comprises a material of high mechanical strength, for example metal, in particular high-quality steel. Preferably the second case portion 5 is in that way in the form of a reflector which on its inside has a layer reflective for solar and/or heat radiation. In FIGS. 1, 2, 4 and 5 it has a smooth or structure-less inside surface of a shape (in particular being parabolic with a single axis) which is suitable for reflecting radiation which enters through the first case portion 4 into the internal space 8 of the casing tube and which is not directly incident on the absorber tube 2 back to same and for focusing it in order thereby to increase the efficiency of the receiver tube 1.

The two end walls 6, 7 preferably comprise the same material as the second case portion 5 and are gas-tightly connected both to the latter and also to the first case portion.

Receiver tubes are used for solar energy conversion not on their own but connected in succession in long rows, in which case their absorber tubes are connected to form a continuous flow path through which the heat carrier medium can flow.

To permit the corresponding connection of the absorber tube 2 to the absorber tubes of the adjacent receiver tubes each of the end walls 6, 7 in the FIG. 1 embodiment has a bore 9, 10 therethrough, in which a through-flow pipe connection 12 and 13 respectively is fitted by means of a thermally insulating sealing element (not shown) in such a way that it projects with a free end out of the gas-tightly closed receiver tube 1 while its end which is in the internal space 8 is connected by way of a compensating element 15 to the end theretowards of the absorber tube 2. The compensating element 15 which serves for compensation of thermally induced or other different changes in length which occur in operation, between the absorber tube 2 and the casing tube, can be for example in the form of a metal bellows and is advantageous in particular when the second case portion 5 and/or the end walls 6, 7 are to be of such a small material thickness that they cannot readily carry the forces which occur upon the above-mentioned different changes in length.

In the embodiment shown in FIG. 2 the through-flow pipe connections 17, 18 in the end regions of the receiver tube 1 are passed out not through the end walls 6, 7 but through bores 20, 21 arranged in the “bottom” region of the second case portion 5, that is opposite the first case portion 4. Here too there are provided heat insulators (not shown) to prevent as far as possible a transfer of heat from the pipe connections 17, 18 to the second case portion 5. The pipe connections 17, 18 are connected by way of pipe bends 23, 24 to the ends of the absorber tube 2, which provide angle configurations in each case of 90° or more. The overall arrangement is such that it can compensate for different changes in length of the second case portion 5 and the absorber tube 2 due to elastic deformation.

In addition both embodiments in the second case portion 5 have two bores 27, 28 into which a respective valve 30 and 31 in the form of a non-return valve is gas-tightly fitted. To be able to flush the internal space 8 of the casing tube with fresh heat-insulating gas those valves 30, 31 are connected to gas feed and suction removal conduits (not shown). When the internal space 8 of the casing tube is evacuated one such valve is sufficient, which is then connected to a suction removal pump by way of a suction conduit to improve the vacuum.

In the case of the second case portions 5 shown in FIGS. 7 through 10 their concave inside surface that in the assembled condition is towards the absorber tube is not smooth but is provided with raised, that is to say inwardly projecting, structures 33 which in the illustrated embodiments are formed by radially directed fins, each of which extends parallel to the longitudinal axis of the second case portion 5 over the entire length thereof.

Those fins which are formed in one piece with the second case portion 5 or are fixedly connected thereto and which can also have reflecting and in particular mirrored surfaces serve to deflect at least a considerable part of the light which is not focused onto the absorber tube 2 by the trough collectors (not shown) for example because of deviations from the ideal parabolic shape or because of imperfect orientation and which is therefore incident on the inside of the second case portion 5, towards the absorber tube by single and/or multiple mirroring, in order thereby to achieve an additional contribution to heating of the absorber tube.

As can be seen in particular from FIGS. 8 and 10 the radial dimensions of the fins or the radial heights with which they extend towards the absorber tube from the inside wall of the second case portion 5 can be the same (FIGS. 7 and 8) or different from each other (FIGS. 9 and 10). The latter has the advantage that this involves reduced shadowing of the fins disposed closer to the apex 34 of the second case portion 5, by the fins arranged more towards its edge regions 35.

FIG. 11 shows a perspective view of a short part of a second case portion 5 which over its entire axial length on its concave inside has structures 33 which, in addition to the fins extending in the longitudinal direction in the embodiments shown in FIGS. 7 through 10, include fins which extend in the peripheral direction and which also project upwardly in the radial direction above the inside surface of the second case portion 5 and which include approximately right angles with the fins extending in the longitudinal direction so that a multiplicity of cells of approximately rectangular and in particular square outline are formed. In FIGS. 11 and 12 all those fins are of the same radial extent or height but here, in a similar way to FIGS. 9 and 10, a reduction in height towards the edge regions of the second case portion 5 is also possible.

The same also applies to the structures 33 which are shown in FIGS. 13 and 14 and in which there are no fins extending in a straight line, but a multiplicity of walls projecting in a radial direction beyond the inside surface of the second case portion towards the absorber tube provide honeycomb cells of polygonal outline which in turn cover at least a large part of the inside surface of the second case portion 5.

As FIGS. 16 through 18 show the structures 33 resulting in deviations from the smooth surface of the second case portion 5 do not necessarily have to be raised, that is to say project towards the absorber tube 2. Rather, they can also be formed by recesses in the surface of the second case portion 5, which are directed radially outwardly, that is to say away from the absorber tube 2, and which in the illustrated example are of an approximately circular outline. However other, for example oval, rectangular or polygonal outlines are also possible. The essential consideration is that those structures which can be produced for example by a simple embossing operation contribute to improved concentration of the sunlight on the absorber tube.

Instead of the structures shown in the Figures, which project radially inwardly or outwardly from the inside surface of the second case portion 5, it is also possible to provide other structures which afford secondary concentration of the solar radiation towards the absorber tube 2. Thus it is possible for example for structures which are wave-shaped, triangular, trapezoidal or rectangular in cross-section to be superimposed on the wall of the second case portion, that in principle is concave when viewed from the absorber tube. 

1. A receiver tube (1) for a solar trough collector which focuses the solar radiation incident thereon towards a focal line, wherein the receiver tube (1) comprises: an absorber tube (2) which in the mounted condition of the receiver tube (1) extends on the focal line of the trough collector and which is in the form of part of a flow path for a heat carrier medium, and a gas-tightly closed casing tube, in the internal space (8) of which the absorber tube (2) extends, wherein the flow path for the heat carrier medium is passed outwardly through the wall of the casing tube in the end regions thereof, wherein the casing tube includes at least two case portions (4, 5) which in the mounted condition of the receiver tube (1) extend in the direction of the focal line and are gas-tightly connected together along generatrices, the first case portion (4) which in the mounted condition is towards the trough collector comprises a material which is transmissive for the solar radiation, the second case portion (5) comprises a fracture-resistant material, and the flow path for the heat carrier medium is passed outwardly through a part of the wall of the casing tube, that is not formed by the first case portion (4).
 2. A receiver tube as set forth in claim 1 wherein the two case portions (4, 5) extend over the entire length of the receiver tube (1).
 3. A receiver tube as set forth in claim 2 wherein the second case portion (5) is so shaped that it reflects at least parts of the solar radiation incident on its inside onto the absorber tube (2).
 4. A receiver tube as set forth in claim 3 wherein the second case portion (5) on its inside towards the absorber tube (2) has structures (33) which differ from a smooth surface and which reflect solar radiation incident thereon in such a way that it is deflected towards the absorber tube (2).
 5. A receiver tube as set forth claim 4 wherein the structures (33) differing from a smooth surface are raised structures (33) which are directed radially towards the absorber tube (2) and which are formed by fins which extend in the longitudinal direction of the receiver tube (1) and which are so mounted at the inside wall of the case portion (5) that they project in the radial direction towards the absorber tube (2).
 6. A receiver tube as set forth in claim 1 wherein at each of its two axial ends the casing tube is closed by an end wall (6, 7) extending substantially perpendicularly to the axis of the receiver tube (1).
 7. A receiver tube as set forth in claim 6 wherein the end walls (6, 7) comprise the same material as the second case portion (5).
 8. A receiver tube as set forth in claim 1 wherein it includes two thermally insulatedly mounted through-flow pipe connections (12, 13; 17, 18) for passing the flow path for the heat carrier medium through the wall of the casing tube.
 9. A receiver tube as set forth in claim 8 wherein the through-flow pipe connections (12, 13) extend through the end walls (6, 7) coaxially with respect to the absorber tube (2) and are respectively connected to the absorber tube (2) by a compensating element (15, 16) providing for compensation of different changes in length of the casing tube and the absorber tube (2).
 10. A receiver tube as set forth in claim 8 wherein the through-flow pipe connections (17, 18) extend through the second case portion (5) of the casing tube transversely relative to the longitudinal extent of the absorber tube (2) and are connected to the axially extending part of the absorber tube (2) by pipe bends (23, 24) so bent that they provide for compensation of different changes in length of the casing tube and the absorber tube (2).
 11. A receiver tube as set forth in claim 1 wherein it includes a valve arrangement (30, 31) which extends 